1. Field of the Invention
The present invention relates to a constant current circuit that generates a predetermined constant electric current.
2. Description of the Related Art
In small information terminals such as a portable phone and a PDA (Personal Digital Assistance) of recent years, there are devices that require a voltage higher than the output voltage of the battery such as a light-emitting diode (Light Emitting Diode, hereinafter referred to as a LED) used as a back light of a liquid crystal, for example. In these small information terminals, a Li ion battery is often used, and the output voltage thereof is typically about 3.5 V, and is about 4.2 V in the fully charged state. However, the LED requires a voltage higher than the battery voltage as the driving voltage thereof. In this manner, when a voltage higher than the battery voltage is required, the battery voltage is boosted with use of a power supply apparatus of boosting type using a switching regulator, a charge pumping circuit, or the like, thereby to obtain a voltage needed for driving a load circuit such as a LED.
In driving the LED with such a power supply apparatus, a constant current circuit is connected on a path for driving the LED and, by keeping the electric current flowing through the LED to be constant, the control of the luminescence brightness thereof is stabilized. At this time, there are cases in which a method is adopted such that the voltage at the connection point of the LED and the constant current circuit is monitored, and the driving voltage of the LED is generated so that this voltage will be a constant value (See Patent Document 1).
[PATENT DOCUMENT 1] Japanese Patent Application Laid-Open (JP-A) No. 2004-22929
Here, in order to achieve stable operation of the constant current circuit connected to the cathode terminal of the LED, the transistor constituting the constant current circuit must operate in a constant current region. Here, the constant current region of a transistor refers to an active region in the case of a bipolar transistor, and refers to a saturation region in the case of a field effect transistor (hereinafter referred to as FET). The transistor constituting the constant current circuit is disposed in series between the cathode terminal of the LED and the grounded terminal. In order that this transistor may operate in the constant current region, the cathode terminal of the LED must be kept at a constant voltage or above.
Assuming that the voltage needed for stable operation of the constant current circuit is Vcs and the current flown through the LED is Iled, an electric power of Pcs=Vcs×Iled is consumed in this constant current circuit. Since the current flown through the LED is a value determined by the luminescence brightness of the LED, the voltage Vcs needed for stable operation of the constant current circuit must be lowered in order to reduce the electric power consumed in the constant current circuit.